The editor’s choice for our September issue is ‘Short-term effects of moderate severity disturbances on forest canopy structure‘ by Dennis Choi et al. Here, Associate Editor Tommaso Jucker discusses the importance of this research:
The launch of the Landsat satellite program in the 1970s forever changed our view of Earth’s biosphere – its oceans, its grasslands, its deserts, its mountains, and perhaps mostly notably its forests. Suddenly, we were able to not only put these vast ecosystems on the map, but also track, more or less in real time, how they were changing at a global scale. In particular, Landsat transformed our ability to capture and quantify so called stand-replacing disturbances: events that cause all canopy dominant trees in a particular forest patch to die – think clearcuttings, large wildfires, windthrows, catastrophic droughts, and bark beetle outbreaks. It’s hard to overstate the importance of this breakthrough. These big disturbance events are critical to understanding the role that forests play in regulating carbon cycling on land, but almost impossible to study using traditional field based inventories due to their sporadic nature.
However, one thing that Landsat and its more recent orbiting siblings aren’t particularly helpful with is tracking how forests recover from less severe disturbances that only affect a portion of trees in the stand, while leaving others alive. Instinctively we may think of these moderate severity disturbances – which include things like host-specific pathogens, defoliating insects, understory wildfires and lightning strikes – as being of secondary importance. But the truth is that in many systems they are the primary driver of tree mortality. So capturing these moderate disturbances and the fingerprint they leave on structure and dynamics of forest ecosystems is critically important, especially in a rapidly changing world where novel combinations of disturbance agents are putting forests under growing pressure.
Step in Choi and co, who in their recent paper develop a novel approach for using repeat airborne LiDAR to track the impacts of a range of different types of moderate severity disturbances on forests across the Eastern US. Airborne LiDAR is a remote sensing technology that uses laser scanning to generate highly detailed and accurate 3D models of forest canopies across whole landscapes. By repeating these scans over time – five scans spread over a 6–8 year period in the case of Choi et al.’s study – we can begin to quantify not just the initial impacts of disturbance on forest canopy structure, but also track its subsequent recovery (or lack thereof). This makes airborne LiDAR the perfect tool for characterising the impacts of moderate severity disturbances on forest canopies, allowing us to shine a new light on what was before just a sea of green.
By combing airborne LiDAR with data from a network of forest plots, Choi and colleagues were able to compare the effects of different types of moderate severity disturbances at five sites belonging to the National Ecological Observatory Network (NEON). These disturbances range both in their intensity – the proportion of trees they suppress or kill – and their modality. Some, like beech bark disease and emerald ash borer, can take years to kill the trees they infect, putting forests under a sustained disturbance pressure. Others, like the spongy moth, kill or maim trees much more rapidly in a single pulse event. Choi and colleagues were able to meticulously catalogue the impacts of these different types of moderate severity disturbance on the 3D structure of forest canopies. In doing so they showed that irrespective of the agent of disturbance, at low disturbance intensities forest canopies were able to recover quickly, while more intense disturbances inhibited height growth and subsequent forest recovery. Moreover, they also found that when it comes to untangling the relationship between disturbance and forest structure, things are more complicated (and interesting) than one might initially suspect. Yes, moderate severity disturbances do reshape the 3D structure of forest canopies. But it is also the case the 3D structure of the canopy itself modulates the severity of the disturbance, with more structurally complex canopies seeming to exhibit greater resilience to certain types of disturbance.
Choi et al.’s paper is one of the best examples I’ve seen showcasing how repeat LiDAR can help us better understand the ways in which disturbances shape forest canopy dynamics. But my guess is that there is plenty more to come, especially as access to these repeat datasets continues to grow. NEON, for instance, may have gotten off to a rocky start (see here and here for a brief history), but as more and more data from this behemoth of a project has come online, we’ve begun to see some bright lights at the end of the tunnel. Ecologists from a wide variety of backgrounds have begun to coalesce into a community that is keen to leverage the full potential of the platform (e.g., here and here), often in ways that are new for our field (e.g., see here and here for community-led data challenges built around NEON). My hope is that NEON and other similar networks around the globe will prove a catalyst for more of this type of collaborative and creative research, in a similar way to what Landsat and other satellite programs have done in recent decades.
Read the full article online: Short-term effects of moderate severity disturbances on forest canopy structure
